Slow worm (Anguis fragilis)

This legless lizard is common and widespread in Scotland, but is declining because of habitat loss.

Slow worm amongst forest floor litter in Glen Affric.
The female slow worm, seen here at Dundreggan, is distinguished by the black stripe along the spine.
Young slow worm in Glen Affric.

Worldwide distribution

The slow worm occurs throughout much of Europe, from Britain, southern Scandinavia and the Iberian Peninsula eastwards to Greece, Turkey and the countries surrounding the Black Sea. It is also native to the Baltic states of Estonia, Lithuania and Latvia, Russia, northern Iran and parts of North Africa, including Algeria and Tunisia. It is absent from many of the Mediterranean Islands and Ireland.

Distribution in Scotland

The slow worm is found in most parts of mainland Scotland, from Dumfries and Galloway and the Borders northwards to Caithness and Sutherland. It is more common in the south and west, and its distribution becomes patchier in the east, northeast and particularly in the far north. It is present on some of the larger western islands such as Lewis, Harris and Skye, but does not occur on most of the smaller ones, nor on Orkney or Shetland.

Because it is a common and widespread species, the slow worm is not considered to be at risk at an international level, and it does not feature on the IUCN Red List of Threatened Species. In the UK, however, the slow worm is protected under Schedule 5 of the Wildlife and Countryside Act, which makes it illegal to intentionally kill, injure or sell the species. It has also been added to the UK’s Biodiversity Action Plan and the Scottish Biodiversity List as a priority species for conservation, because of declines in its population due to habitat loss.

The slow worm, which is also sometimes called the blind worm, is a limbless member of the Anguidae family, and is the reptile most commonly seen in the UK. The only other member of the family, the Peloponnese slow worm (Anguis cephalonnica), is endemic to parts of Greece.

Despite its common name, the slow worm is neither slow in its movements, nor a worm. Sometimes mistaken for a snake, it is in fact a lizard that has evolved over time to be legless. It is differentiated from snakes by several anatomical features, including the presence of eyelids and ear openings (which snakes don’t have) and by its ability to shed its tail when threatened (which no snake is able to do).

An adult slow worm is up to 50 cm long and covered in small scales that are shiny and smooth, and do not overlap each other. This gives its body an almost glass-like appearance and aids its movement through soil. The male and female can be differentiated by their colouration when they are mature. Females often have a dark vertebral stripe along their back, and the sides can be patterned with dark brown or black, whereas the males tend to be more uniform in colour and sometimes have distinct blue spots on their body. Males also have proportionally larger heads. This variation between the genders is known as sexual dimorphism.

Like many lizards, the slow worm has a defence mechanism against predation, whereby it loses its tail when threatened by a predator. The scientific name for this process is autotomy, and it is achieved by the slow worm squeezing a muscle that breaks one of the tail vertebrae in half. The detached tail will wriggle and twitch, thereby distracting a predator and allowing the slow worm to escape. A new tail will subsequently grow as a replacement, although it will be slightly shorter than the original. The specific part of the slow worm’s scientific name, ‘fragilis’, meaning ‘fragile’, is derived from this ability to sacrifice its tail in order to protect the rest of its body (including all its vital organs) from predation.

The slow worm lives in a range of habitats such as the edges of woodlands (including the Caledonian Forest), heathlands, tussocky grasslands, uncut grassy meadows, and gardens and allotments. It is often found in compost heaps as it benefits from the warmth generated by the decaying vegetation, and feeds on the slugs and snails that also live there. Although it is diurnal (meaning that it is active during the day), it is quite a secretive species, spending much of the day out of sight under stones or pieces of wood, or in a burrow in the ground.

The slow worm is a carnivore, and it feeds on a variety of invertebrates including snails, slugs such as the European black slug (Arion ater), worms, spiders and insects. It has backward-pointing, curved teeth that enable it to firmly hold prey such as slugs, which are wet and slippery to the touch. The tongue is notched rather than forked as with snakes, and it has an excellent sense of smell.

As an ectothermic reptile that depends on ambient heat to maintain its body temperature, the slow worm hibernates during the winter, typically from October to early March. Hibernation takes place underground, either as single individuals or sometimes communally with other slow worms.

After emergence from hibernation in March, slow worms can be seen basking in sunshine in the early morning and late afternoon, as they come into condition for breeding, which takes place in May. 

The slow worm sheds its skin periodically throughout its life, and the old skin falls away in a number of sections, rather than as an intact entire length, which is the case with snakes. It is a long-lived species, with individuals surviving for up to 30 years in the wild, while one in captivity lived to the age of 54.

Courtship occurs with a male biting and holding a female by her head or neck, and the couple intertwine their bodies, sometimes for several hours. After a gestation period of 3-4 months, the female gives birth to an average of eight young between mid-August and mid-September. The slow worm is ovoviviparous, which means that rather than laying eggs, the female gives birth to live young encased in a thin membrane that ruptures soon after birth. A newborn slow worm is 7-10 cm in length, and growth is relatively slow, with it taking 6-8 years for an individual to reach its full size. Males reach sexual maturity after 3-4 years while the females take 4-5 years to become capable of reproduction.

Like most organisms, the slow worm has its own suite of predators and parasites, which help to shape both the lives of individual animals and the evolution of the species as a whole. In the UK the slow worm is preyed upon by the adder (Vipera berus), hedgehog (Erinaceus europaeus), badger (Meles meles), red fox (Vulpes vulpes), crows (Corvus spp.), rats (Rattus spp.), and various birds of prey. It is also eaten by the introduced non-native pheasant (Phasianus colchicus), and in urban locations, the domestic cat (Felis catus) is a significant predator of the slow worm.

Common endoparasites, or internal parasites, of the slow worm in the UK include two nematodes (Neoxysomatium brevicaudatum) and (Entomelas entomelas). Elsewhere in its range, 7 different species of parasitic worms have been recorded as occurring in the slow worm.

Because of its utilisation of compost heaps, the slow worm can have a beneficial effect for people, by feeding on the slugs and snails that are otherwise the bane of many gardeners.

Alan Watson Featherstone

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